Feline immunodeficiency virus (FIV), formerly called feline T-lymphotrophic lentivirus, was first isolated in 1986 from a large multiple cat household in Petaluma, Calif. (Pederson et al., Science (1987) 235:790). FIV infects cats to produce an AIDS-like syndrome. Although FIV is morphologically and pathologically similar to the human immunodeficiency virus (HIV), it has been shown to be antigenically distinct from HIV. Like HIV, once a cat becomes infected with FIV, the disease progresses from a primary infection (viraemia, fever, general lymphadenitis) to a lengthy asymptomatic phase, followed by severe impairment in immune function caused by a reduction in CD4 lymphocytes, and resulting in heightened susceptibility to secondary infections and ultimately death.
FIV has been classified as a member of the subfamily Lentiviridae in the family Retroviridae, the family that includes human and simian immunodeficiency viruses, equine infectious anaemia, maedi visna of sheep and caprinearthritis encephalitis viruses (CAEV). The genome of FIV is organized like other lentiviruses with three long open reading frames corresponding to gag, pol and env (Talbott et al., Proc. Natl. Acad. Sci. (1989) 86:5743; Olmsted et al., Proc. Natl. Acad. Sci. (1989) 86:2448). The gag gene codes for the major structural components of the virus, the env gene codes for the envelope glycoprotein, and the pol gene codes for the polymerase protein.
The gag gene is expressed as a 55 kD polyprotein that is processed into three subunits: a p15 matrix protein, a p24 capsid protein, and a p10 nucleocapsid protein. The pol gene encodes three proteins: the protease, reverse transcriptase and a p14.6 protein of unknown function. Autoprocessing by the protease portion of the gene gives rise to all three proteins of the pol region. Additionally, the protease is responsible for the processing of the gag precursor. The pol gene is expressed as a gag-pol fusion protein. The envelope gene is expressed as a 160 kD glycoprotein, gp160. The antigenicity of the FIV core proteins is similar to other lentiviruses.
Several independent viral isolates have been prepared across the world, and a certain number of studies have been carried out in order to demonstrate the structure of the isolated strains: the American strain Petaluma, Talbott et al. Natl. Acad. Sci. USA, 1989, 86, 5743–5747; Philipps et al., J. Virol., 1990, 64, 10, 4605–4613), the Japanese strains (the TM1 and TM2 strains), Miyazawa et al., Arch. Virol., 1989, 108, 59–68, and the Swiss isolates (FIVZ1 and FIVZ2), Morikawa et al., Virus Research, 1991, 21, 53–63.
The nucleotide sequences of three proviral clones derived from American FIV isolates (Petaluma strain) have been described (clones FIV34TF10, FIV14 and isolate PPR) (Olmsted, et al. 1989; Philipps et al., 1990; Talbott et al., 1989) and compared with two Swiss isolates (Morikawa et al. 1991). This comparison led Morikawa et al. to specify the presence of certain conserved regions and certain variable regions within the env gene of FIV. French strains have also been isolated (strains Wo and Me)(Moraillon et al., 1992, Vet. Mic., 31, 41–45).
The virus replicates optimally in blood mononuclear cells and has a tropism for T-lymphocytes, peritoneal macrophage, brain macrophage and astrocytes. In common with other retroviruses, the genetic material of FIV is composed of RNA and the production of a DNA copy of the viral RNA is an essential step in the replication of FIV in the host. This step requires the enzyme reverse transcriptase that is carried into the host by the invading virus. The DNA version of the viral genome is inserted into the genetic material of infected host cells in which it continues to reside as a provirus. This provirus is replicated every time the cell divides and can code for the production of new virus particles. Cells infected with FIV remain infected for the duration of their lifespan.
The virus appears to be spread naturally by horizontal transmission, predominantly by bite wounds from an infected cat as these animals shed appreciable amounts of virus in saliva (Yamamoto et al., Am. J. Vet. Res. 1988, 8:1246). Vertical transmission has been reported, but is rare.
Current diagnostic screening tests for FIV infection detect serum antibody (Ab) to FIV. Virus detection kits are also available but not as prevalent. A number of diagnostic tests are available to determine the presence of FIV antibody in infected animals. For example, PetChek® FIV Ab test kit and the SNAP® Combo FeLV Ag/FIV Ab test kit (IDEXX Laboratories, Westbrook, Me.) are immunoassay based diagnostic tests for FIV infection.
Detecting FIV infection is becoming increasingly important as studies reveal FIV infection is widespread worldwide. As vaccines have been developed in attempt to combat the disease, it is even more important to be able to detect the effectiveness of a vaccine and to discriminate between vaccinated cats versus naturally infected cats.